1. Field of the Invention
This invention relates generally to a method and apparatus for cleaning molds and, more particularly, to a method and apparatus for cleaning the top and bottom mold portions used for encapsulating the integrated circuit and lead portion of a lead frame assembly with plastic material by ion bombardment from a reverse sputtering operation.
2. Description of the Prior Art
The prior art includes many different methods and types of apparatus for producing integrated circuits and for producing lead frames for carrying the integrated circuits. In such systems, the integrated circuit is positioned, generally centrally, within the lead frame to form a lead frame assembly and the input/output leads of the integrated circuit itself are electrically coupled to the leads of the lead frame assembly. The central portion of the lead frame assembly including the integrated circuit portion and the leads into and out from the integrated circuit are then encapsulated in plastic, ceramic, or the like so that only the lead pins or terminals protrude or extend from the encapsulating material.
Various methods and types of apparatus can be used for performing the encapsulation operation. A typical in-line, continuous system includes positioning the previously-formed integrated circuit within the central portion of the lead frame and effecting the necessary electrical connections. The lead frame assembly is then positioned in a bottom mold portion and the top mold portion is positioned thereover. The plastic material is fed or supplied into the mold cavity and heat and pressure are applied to reduce the plastic material to a molten state and cover the central integrated circuit and lead portion of the lead frame assembly. The encapsulated lead frame assembly is then cured and the top mold portion separated from the bottom mold portion prior to removal of the lead frame assembly. The top mold portion and bottom mold portion are then subjected to some type of physical/mechanical and/or chemical cleaning operation prior to reuse for subsequent encapsulation operations. The lead frame assembly removed from the bottom mold portion is then forwarded for further processing or the like.
The prior art techniques for mechanical and/or chemical mold cleaning often produce many significant problems, particularly when a continuous in-line operation is to be maintained.
The molds used in an encapsulation operation may be made from various types of metal although stainless steel is generally preferable. The cleaning of the metal mold portions generally involve the removal of undesirable encapsulating materials, dirt, dust, greast, oil, etc. lying on the various mold surfaces. The cleaning must include not only the removal of all visible dirt from the surfaces, but also the subsequent removal of all contaminates and impurities physically stuck or adhered on the surface such as oil, grease, dust, and the like or those contaminates or impurities resulting from a chemical reaction such as oxides, sulfides, and the like. The degree of cleanliness must be very high when dealing with semiconductor circuits since contamination prior to encapsulation often results in an inferior or non-functional product.
The oxides and other similar surface layers can be removed by mechanical and/or chemical methods such as abrasive blasting, wire brushing, pickling and etching. The cleaning of oils and greases depends upon their particular nature and wheather or not they are soap-forming oils and greases of animal or vegetable origin, or mineral oils which do not form soaps. The soap-forming oils and greases can be removed by transforming them by hydrolysis into fatty acids and by reacting those acids with alkaline solutions to obtain water soluble soaps. The mineral oils can be removed by dissolving them in organic solvents and, in particular cases, they can be washed with alkaline solutions containing detergents. Since the nature of the contaminates is usually unknown, a single reliable cleaning operation must include at least the successive steps of degreasing with organic solvents followed by alkaline degreasing.
The sequence of cleaning operations begins generally with mechanical cleaning and has usually been followed by pickling, detergent cleaning and degreasing. The mechanical cleaning methods often used in the prior art for the purpose of removing scale, rust, plastic residue, and the like often included blasting or wire brushing the mold surface. This could obviously result in damage to the mold portions themselves as well as in forming surface defects in the mold which could cause damage to the encapsulated circuit or in reducing the life of the mold.
A pickling operation is the chemical removal of oxides, sulfides, CH.sub.4 and other surface layers, leaving the cleaned part with a generally bright metallic appearance. The particular pickling solution used depends upon the particular metal used in the mold portions and in the primary substances being removed. After pickling, the part must always be thoroughly rinsed and subsequently neutralized in an alkaline bath and then dried with hot oil-free air. This results in a great loss in time, an increase in expense, and the destruction or at least undermining of any proposed continuous in-line molding operation.
Alkaline detergent cleaning is performed by immersion or by electro-cleaning processes. The immersion cleaning method is usually used with hot solutions. For ferrous metals, the solution generally contains sodium hydroxide, soaps and wetting agents and for electrocleaning, an alkaline solution can be used with the metal to be cleaned used as the cathode or the anode and a tank being the second electrode. With anodic cleaning, oxygen is liberated on the surface of the metal being cleaned, and the process requires relatively low voltage. With cathodic cleaning, hydrogen is liberated on the clean surface, and the process requires a similar relatively low voltage. For stainless steel and the like, anodic cleaning is recommended but again, it is very costly, slow and detrimental to maintaining a continuous in-line molding process.
Solvent cleaning has been done by using the solvent in a liquid or in a vapour state. Liquid cleaning can be done with benzine, xylene, or inflammable solvents such as carbon tetrachloride, trichlorethylene, perchlorethylene, or dichlorethylene.
Vapor degreasing is much more effective than liquid solvent cleaning, but again, it requires additional cost, loss of time, and complex apparatus. The solvent must be heated to boiling and the parts to be cleaned must be hung within the chamber suspended in the hot vapor which condenses on the metal surfaces, dissolves the surface contaminates, and then flows or falls back into the solvent container. Systems have also been proposed which utilize high-pressure liquid solvent spraying or the like which may increase the overall time of the operation and further add to the expense.
Therefore, the various mechanical and/or chemical cleaning methods of the prior art do not produce the quality of cleanliness required when dealing with semiconductor circuits; are not conducive to continuous in-line molding operations; are too expensive; too mechanical and/or electrically complex; require excess maintenance; and result in cumulative damage to the relatively expensive top and bottom mold portions.
The phenomenon of DC sputtering or cathodic sputtering refers to the dislocation or removal of atoms or molecules from the surface of a material by the impact energy of gas ions which are accelerated in an electric field. Cathodic sputtering is established by the creation of a glow discharge or plasma between an anode and a cathode wherein the current therebetween is composed of electron flow to the anode and positive ion flow to the cathode. The ions are created by the ionization of gas molecules existing within the flow discharge region between the anode and cathode. The ionization results from the collision of gas particles with the electron flow from the cathode to the anode. Sputtering is used extensively for depositing thin films of semiconductor material, metal, and the like on various surfaces.
The removal of surface contaminates by cathodic sputtering is known in the art, at least for theoretical or laboratory-type operations, and is referred to as "reverse sputtering" since it is the opposite of the process of cathodic sputtering itself wherein substances are deposited onto the surface of a material. For example, reverse sputtering removes material from a surface as observed in electric-arc-inert-gas welding applications wherein contaminates are removed from the surface of the materials to be welded prior to the actual weldment.
Reverse sputtering has been used in the prior art to clean relatively large surface areas of semiconductor material as a preliminary step in the manufacture of semiconductor devices such as photoelectric cells, and the like. Furthermore, reverse sputtering has been used to clean the actual apparatus used in cathodic sputtering operations and in various applications such as cleaning accelerators, storage rings, and plasma machines.
The prior art also teaches relatively delicate apertured masking techniques for selectively cleaning small selected areas of a material while shielding or masking other areas to prevent ion bombardment of the shielded or masked surface.
Because of the extreme delicacy of the method and apparatus of sputter cleaning or reverse sputtering to clean semiconductor circuits, and the like, and due to the inaccuracy of masking techniques, prior art attempts have usually resulted in a return to known physical abrasive cleaning techniques and/or chemical cleaning techniques such as etchants, abrasive microcloth, and the like. The use of these methods, however, does not achieve a uniform cleaning of the surface area, and such methods are particularly ineffective, unsatisfactory, or difficult with mold apparatus wherein the mold recesses present relatively small, intricate, well-defined areas of restricted size and confinement which are extremely difficult to clean by such prior art methods. Furthermore, such cleaning techniques often result in damage to the work piece and hence damage to the product produced in the mold assembly.
The method and apparatus of the present invention solve relatively all of the problems listed above and provide a relatively low cost, extremely simple, easy to maintain, highly efficient, system usable in continuous in-line molding or encapsulation systems.